PARIS: Naomi Osaka expects Thursday's French Open second-round meeting with a resurgent Victoria Azarenka to be her toughest challenge this year as the world number one struggles to find the form that won her the last two in January Grand Slam titles.

Osaka has had a series of inconsistent results amid mounting pressure since she followed up her US Open success with the Australian Open title and was a bundle of nerves during her opening match in Paris on Tuesday.

The Japanese lost the first set tamely before fighting back to overcome Slovakia's Anna Karolina Schmiedlova 0-6 7-6(4) 6-1, but is well aware that former world number one Azarenka could punish her for another slow start.

"She's been playing really well recently," Osaka said of the Belarusian, who is showing signs of returning to her best following the birth of her son Leo in 2016 and an ensuing custody battle with her former partner.

Osaka eased to a straight-sets victory when the pair met in Rome last year, but the 21-year-old does not foresee another one-sided contest.

"There is no way that it's going to be a similar match to the one I played her in Rome last year," Osaka said. "So I'm expecting one of the hardest matches of the year."

Azarenka beat former champion Jelena Ostapenko 6-4 7-6(4) to advance to her third career meeting with Osaka.

"It's going to be exciting. I had some good wins in the last couple weeks against top 10 players, so I feel that I'm ready to play against top players," Azarenka said.

In other matches on the main show courts, Serena Williams takes on Japan's Kurumi Nara while men's top seed Novak Djokovic meets Henri Laaksonen of Switzerland.

Defending champion Simona Halep faces Magda Linette of Poland.